1. Field of the Invention
The present invention relates to a method of manufacturing by press forming a thin-plate glass with a thickness of about 3 mm or less which is used as a substrate of a magnetic recording medium, a magneto-optic recording medium, an optical recording medium and another information recording medium or as a plate glass for a camera filter, mask blanks or the like, also relates to a method of manufacturing a glass substrate for an information recording medium and further relates to a magnetic recording medium.
2. Description of Related Arts
A glass substrate has been increasingly used as a substrate for a magnetic recording medium or another information recording medium lately. Further, as a method of manufacturing the glass substrate, instead of cutting out of a plate glass, molten glass is directly pressed by using a forming die, that is, a direct pressing method is used.
A prior-art direct pressing method is disclosed in Japanese Patent Application Laid-open No. Hei 5-105458. In this method, a forming die with a die release agent layer formed on a forming face is used. A glass material is pressed for a sufficient period of time until the glass material is at a softening point or a lower temperature and is thermally in balance with upper and lower dies, to form a disc-like glass product having a final configuration with a small warp.
However, in the method described above in which the press forming is performed for a sufficient time until the glass material has a softening point or a lower temperature and brought in thermal balance with the upper and lower dies, the press forming takes much time. Thus, mass productivity is disadvantageously deteriorated.
Also, in the method disclosed in Hei 5-105458, it is difficult to control temperatures completely. The warp can be minimized only to some degree. The glass product which is obtained in the manufacture method has been heretofore able to be used as the magnetic recording medium substrate. Recently, however, a demand for densification of the magnetic recording medium has been increased. A substrate with a smaller warp than a conventional substrate is requested for. Especially, the magnetic recording medium substrate which can be used on an MR head is requested to have a high flatness. Therefore, the prior-art method may not be desirable.
Also in the method disclosed in Hei 5-105458, when manufacturing a glass substrate for the MR-head magnetic recording medium, the glass substrate needs to be finally ground and polished to conform to predetermined specifications. The grinding is performed by using a grinding plate while a pressure is applied to both side faces of a thin-plate glass. The thin-plate glass with a warp is deflected when ground. Thus, problem is that when pressure is released from both sides of the ground thin-plate glass, the thin-plate glass is again warped. To solve the problem, when the thin-plate glass is ground, the pressure to be applied thereto needs to be constantly adjusted finely so that the thin-plate glass fails to be deflected. This lengthens the grinding time. Also in this respect, the mass productivity of the thin-plate glass with a good flatness cannot be enhanced.
Another direct pressing method is disclosed in Japanese Patent Application Laid-open No. Hei 7-133121. In this method, surface temperatures of pressing faces of upper and lower dies are set to a transition point of a glass to be press-formed or its vicinity. Additionally, while inner-surface temperatures of cylindrical dies are set higher than the pressing-face surface temperatures described above, molten glass is pressed between the upper and lower dies to form a disc-like glass product close to a final product.
Also in this method, however, the obtained glass substrate needs to be ground/polished to conform to the predetermined specifications in the same manner as the method described above. For the same reason as described above, the mass productivity is deteriorated.
An object of the present invention is to provide a method of manufacturing a thin-plate glass having a good flatness with a high mass productivity. Further object is to provide a method of manufacturing a glass substrate for an information recording medium and to provide a magnetic recording medium.
To attain these and other objects, the present invention provides a method of manufacturing a thin-plate glass in which press forming is performed between a lower die onto which molten glass is supplied and an upper die which is opposed to the lower die. The press forming is performed between the lower and upper dies which are kept at a predetermined temperature. The press forming is finished when the inside of the thin-plate glass has a temperature higher than a glass transition point. Subsequently, a warp of the press-formed thin-plate glass is modified. The warp modifying process is finished when the inside of the thin-plate glass has a temperature higher than the glass transition point. To shorten the time for pressing, the press forming and the warp modification pressing are preferably finished when the temperature is higher than a glass softening point. Also, the glass temperature during the warp modifying process is preferably lower than the press forming temperature. Thus, since the pressing is finished at the temperature which is higher than the glass transition or softening points, the press-formed glass basically maintains a configuration given by forming faces of the dies even after released from the dies, and can be slightly deformed by a external force. Each pressing needs a period of time preferably within two seconds, more preferably within 1.8 seconds.
Here, the inside of the glass described above means a glass main portion covered with a surface layer which remarkably radiates heat. The temperature of the inside of the glass influences the configuration maintaining property and deformability.
Also, the thin-plate glass of the invention means a thin-plate glass substrate represented by a glass substrate for a magnetic disc. Typically, the glass has a thickness of 2 to 4 mm and a diameter and a length both of 15 cm or less.
The lower die is designed to successively go through glass gob supply process, press forming process, warp modifying process, formed product taking process and the like. For example, plural lower dies are preferably arranged on the circumference of a turntable, and the turntable is preferably rotated in such a manner that the lower dies go through the processes. The lower dies may be designed to move linearly. Also, a single lower die may be supplied to each of the processes.
On the other hand, the upper die is disposed opposite to the lower die which is positioned in the press forming process. Therefore, the number of the upper dies needs to be at least the same as the number of the lower dies for use in one press forming, but more number of the upper dies may be used. Also, a single upper die may be used on the condition that by removing the heat transferred from the molten glass to the upper die after the press forming, temperature can be controlled in a short time to allow the temperature of the upper die to reach an appropriate temperature at the time of press forming.
Subsequently, the temperature of the forming faces of the upper and lower dies (hereinafter, often referred to as the forming die) needs to be adjusted to a predetermined temperature when the press forming is started.
Here, the predetermined temperature of the forming die means the temperature adequate to form a glass material to a thin plate. The temperature is appropriately determined by glass species, thickness, glass plate size and the like.
Further, in order to adjust the temperature of the upper and lower die forming faces to the above predetermined temperature when starting the press forming, the upper and lower dies are heated or cooled as required.
As means for heating the dies used is a heating method in which plural Nichrome heaters are arranged around the lower die (upper die), an induction heating method for heating the forming die constituted of an electric conductor by supplying electricity to a coil which is disposed to surround the periphery of the lower die (upper die), a method of heating the dies with gas or the like. When plural lower dies (upper dies) are arranged, it is difficult to uniformly heat the lower dies (upper dies) with the Nichrome heaters which are arranged around the lower dies (upper dies). Because there are dispersions in temperature among the Nichrome heaters. Therefore, the induction heating method is preferable because uniform heating can be achieved. In the induction heating, since each lower die (upper die) can be heated with one coil, there is no problem of dispersion in heat supply temperature. By disposing the coil at a constant distance from the lower die (upper die), the forming die can be uniformly heated. Here, during the induction heating, a high-frequency current is preferably supplied to the coil. If a low-frequency current is supplied, a large-sized device is required. Also, since the low-frequency current is in a human audible sound region, a problem of noise arises in some case.
On the other hand, when a single lower die (upper die) is used, there is no problem of dispersion in temperature among the Nichrome heaters. Therefore, the lower die (upper die) can be heated by the Nichrome heaters arranged therearound.
Further, the temperature of the forming die in the press forming rises as compared with before the press forming. To continuously form the glass, the forming die needs to be cooled before the subsequent press forming, so that each product is formed on the equal temperature conditions. Therefore, in addition to the heating means, cooling means is necessary.
As the cooling means used is a method in which water or air is circulated in a hollow portion of the forming die, a method in which water or another liquid is blown to an inner face of the hollow portion of the forming die for vaporization or the like. In the method of blowing and vaporizing the liquid, the forming die can be cooled with the vaporization heat of the liquid. Therefore, a cooling effect can be obtained with a less amount of liquid than the method of circulating the liquid. The method using the vaporization heat of water or the like is preferable not only because the high cooling effect can be obtained but also because a cooling device can be reduced in size. Further, for example in the case where the cooling of the upper die takes so much time that after the forming the upper die cannot be cooled to the predetermined temperature before the next forming, plural upper dies may be arranged. In this case, while either one of the upper dies is used for the press forming, the other upper dies are cooled, so that the cooled upper dies can be successively used for the press forming.
In the present invention, when the press forming is finished, the temperature of the thin-plate glass is higher than the temperature of the forming die. At this time, the thin-plate glass and the forming die do not attain a thermally balanced condition. However, as aforementioned, since the forming die is kept at the predetermined temperature beforehand, the formed and cooled thin-plate glass has a fixed configuration whose warp or another quality is constant. The configuration can be easily ground/polished. Also, the thin-plate glass and the forming die need not to be cooled until they reach the thermally balanced condition. Therefore, the forming time can be shortened.
The temperatures of the upper and lower dies for use in the press forming are determined in accordance with glass type, glass releasability, die damages and the like. The temperature of the upper die is preferably between 250xc2x0 C. and 450xc2x0 C., and that of the lower die is preferably between 350xc2x0 C. and 550xc2x0 C. If the temperature is lower than the lower limitation of the die temperature, the glass will be insufficiently elongated. If it exceeds the upper limitation, the glass will be burnt to adhere to the dies.
The temperature of the upper die is preferably the same as or lower by 50 to 100xc2x0 C. than the temperature of the lower die. Also, the cylindrical die for guiding the lower die and/or the upper die preferably has a temperature close to the lower-die temperature.
The lower-die temperature is preferably controlled in accordance with the warp condition of the pressed, formed and warp-modified thin-plate glass. When the completed plate glass is inwardly warped, by lowering the lower-die temperature, the glass viscosity is lowered and the subsequent warp modifying process can be performed properly. Conversely, when the glass is outwardly warped, by raising the lower-die temperature and increasing the glass viscosity in the warp modifying process, the warp can be properly modified.
Examples of the forming die for use in the press forming may include, in addition to a forming die in which the forming faces of both the upper and lower dies have flat surfaces, a forming die in which the forming faces of both the upper and lower dies have convex surfaces, and a forming die in which one of the upper and lower dies has a convex surface and the other has a concave surface. When both the upper and lower dies have the convex or concave surfaces, the thin-plate glass is obtained in which both surfaces are concave or convex. Even when the warp remains in the thin-plate glass, the glass is hardly deformed by the load of a lapping machine in a coarse grinding process.
Moreover, by designing the configurations of the forming faces in such a manner that the curvatures of upper and lower faces of the thin-plate glass are equal to each other, the polishing areas of the upper and lower faces can be equalized, and the lapping machine loads of the upper and lower faces can be equalized. Therefore, both the upper and lower faces can be set to a desired range of polishing pressures.
To form molten glass into a thin plate, the molten glass needs to be stretched in an outer peripheral direction. Therefore, by sticking a solid lubricant to the forming face of the forming die, the lubricating property of the molten glass is preferably increased. In this case, when forming the thin-plate glass, the forming die receives more heat from the molten glass as compared with when press-forming a thick-plate glass and, therefore, attains a high temperature. Therefore, the solid lubricant is preferably resistant to heat, so that it fails to lose its lubricating property even in a high-temperature region. The heat-resistant solid lubricant is not especially restricted as long as it is superior in heat resistance, but boron nitride (BN) is preferable.
Also, to obtain the remarkably thin-plate glass which is yet superior in mechanical strength, a glass material having a high melting temperature is used in some case. In this case, the temperature of the forming die is also remarkably high. Therefore, the solid lubricant is requested to have a remarkably high heat resistance. Also in this case, BN powder or another high heat-resistant solid lubricant powder is preferably used.
When the heat-resistant solid lubricant is powdered, the lubricant can uniformly stick to the glass forming face. Also, excess lubricant can be easily removed.
The material of the forming die is preferably resistant to heat. Graphite, tungsten alloy, nitride, carbide, another heat-resistant metal or the like is used. Especially, cast iron is preferable because it is superior in strength and durability.
In the invention, press-formed glass bodies are different in quantity of heat radiated through each die, which causes opposite main surfaces of the glass bodies to curve and warp.
To reduce and remove the warp, the warp modifying process of the thin-plate glass is performed. As a method for modifying the warp used is a method in which heat is taken away by blowing air or the like to either face with a higher temperature of upper and lower faces or a method in which by pressing a pair of flat boards against the press-formed glass body, the glass body is flattened by the external force (hereinafter, referred to as the warp modification pressing). When the pressure is applied to the glass body by the pair of flat boards, the upper and lower dies which have been used for the press forming can be used as the boards. The viscosity of the glass during the warp modifying process preferably has a value at which the glass maintains the press-formed configuration when no external force is applied thereto and is slightly deformed when the external force is applied.
The temperature of the flat boards for use in the warp modifying is preferably between 400xc2x0 C. and 650xc2x0 C. If it is lower than 400xc2x0 C., defects arise in the glass. If it exceeds 600xc2x0 C., the glass adheres to the boards. The temperature is preferably lower than the glass temperature by 250 to 20xc2x0 C., and preferably higher than the temperature of the forming press dies by 50 to 200xc2x0 C. This is because during the warp modifying process the glass temperature and viscosity are lower than those at the time of press forming. During the warp modifying, the glass needs to be prevented from cracking.
During the press forming, an amorphous molten glass is formed into a fixed configuration which is given by the inner peripheral forming face of the forming die. However, the warp modification pressing described above does not mean that the press-formed thin-plate glass is worked, but means that the warp of the thin-plate glass worked by the press forming is removed (or reduced) so that the thin-plate glass obtains a flat or substantially flat configuration. During the preferable warp modification pressing, the thin-plate glass flattened between the upper and lower dies is cooled and hardened by the pair of flat boards (or the upper and lower dies) which have a lower temperature than the glass, to modify the warp of the thin-plate glass.
The warp modification pressing is preferably performed when the inside of the thin-plate glass has a higher temperature than the glass transition point. If the warp modification pressing is performed at the glass transition point or lower temperature, the thin-plate glass tends to crack or break because the glass is excessively hard. On the other hand, if the thin-plate glass is excessively soft, the glass will be unfavorably deformed by the warp modification pressing. For this reason, the upper limitation of the temperature at which the warp modification pressing is performed is predetermined.
Even in the case where heat is taken from the face having a higher temperature out of the upper and lower faces of the press-formed thin-plate glass and a temperature difference between the upper and lower faces is reduced to modify the warp, or even in the case where the glass is deformed by the external force to modify the warp, it is important that the pressing faces of the flat boards for use be in contact with the glass as desired. Therefore, in the warp modification pressing, it is preferable to use the flat boards having the configurations conformed to the configuration of the press-formed thin-plate glass.
Here, since the temperatures of the glass and the forming die in the warp modification pressing process are different from those during the press forming, the forming die used in the press forming does not have a forming face configuration matched with the glass configuration because of its difference in thermal expansion from the glass. Therefore, it is necessary to determine the forming face configuration of the forming die in consideration of the configuration change of the forming face by the thermal expansion in the pressing temperature and the warp modification pressing temperature. For example, during the warp modification pressing, it is necessary to use the flat boards which are different in the curvature of the forming face from the forming die used in the press forming.
As described above, in order to select the configurations of the press forming upper die and the warp modification pressing flat boards in accordance with the temperature of the forming die and/or the glass during the process, it is preferable to release the upper die once from the glass after the press forming, and press the glass with a separate warp modifying flat board which has a desired forming face configuration in accordance with the temperature during the warp modification.
Here, when a pressure bearing portion described later is formed during the press forming, the warp modification pressing may be performed on both of or either one of the pressure bearing portion and a central portion surrounded by the pressure bearing portion.
Moreover, when the warp modification pressing is performed on the central portion, substantially the entire surface of the surrounded portion may be pressed, or only the selected predetermined portion may be pressed. Examples of the selected predetermined portion include a portion whose flatness is particularly to be enhanced among main surfaces of the thin-plate glass, and a main portion whose flatness determines the flatness of the entire glass.
Moreover, by performing the warp modification pressing to the pressure bearing portion, in processes such as a lapping grinding process described later the position of the board to a polishing plate is easily stabilized when the pressure by the lapping machine is received from both surfaces, and as a result the flatness after the grinding is enhanced.
For the thin-plate glass which is completed through the press forming and warp modification pressing according to the invention, warp is completely removed in some case, and warp remains in another case. If the warp can be mechanically removed by grinding and polishing, the warp may remain in the pressed or finished product.
If the warped glass substrate is simply and normally ground and polished, the finished product will still be warped. Therefore, the grinding and polishing of the warped pressed product needs to be devised.
To realize a good grinding method, the pressed product may be ground while the curvature of the pressed product is inhibited from being deformed by a load of a lapping machine. Specifically, by gradually increasing the load of the lapping machine with an elapse of time from the start of the grinding, first a flat glass substrate is formed. Subsequently, by further increasing the load, the glass substrate is ground to have a predetermined thickness.
Alternatively, to inhibit the curvature of the pressed product from being deformed, instead of forming the pressed product in a simple disc configuration, a pressure bearing portion for supporting the pressure of the lapping machine may be formed on a peripheral portion of the pressed product during the press forming. In this case, since the pressure of the lapping machine is received by the pressure bearing portion, the curved portion can be easily inhibited from being deformed.
The above grinding and polishing method for flattening the warped glass substrate can be applied to the pressed product which is obtained by the pressing method which is disclosed in Hei 5-105458 or 7-133121.
Moreover, as described above, for the thin-plate glass whose both surfaces are convex or concave, the deformation by the load of the lapping machine hardly occurs.
The thin-plate glass which is obtained by the manufacture method as described above is ground, polished and mechanically worked otherwise to form, for example, a glass substrate for an information recording medium. The grinding and polishing process is generally or roughly constituted of (1) rough machining (coarse polishing), (2) sanding (precise grinding, lapping), (3) first polishing and (4) second polishing (final polishing). If necessary, (1) the rough machining (coarse polishing) may be omitted.
Further, the glass substrate for the information recording medium constitutes a magnetic recording medium when a substrate layer, a magnetic layer, a protective layer and a lubricating layer are successively laminated on the glass substrate.
Here, as the material of the glass substrate for the magnetic recording medium used is, for example, aluminosilicate glass, soda lime glass, soda alumino-silicic acid glass, alumino-borosilicate glass, borosilicate glass, quartz glass, chain silicate glass, or crystallized glass or the like. Further preferably, the following glass composition is used.
(1) Crystallized Glass 1
The crystallized glass contains in percentages by weight 60 to 87% of SiO2, 5 to 20% of Li2O, 0 to 5% of Na2O, 0 to 10% of K2O, 0.5 to 10% in total of Na2O and K2O, 0.5 to 7.5% of MgO, 0 to 9.5% of CaO, 0 to 15% of SrO, 0 to 13% of BaO, 0 to 13% of ZnO, 0 to 10% of B2O3, 0 to 10% of Al2O3, 0.5 to 8% of P2O5, 0 to 5% of TiO2, 0 to 3% of ZrO2, 0 to 3% of SnO2, 0 to 2% in total of As2O3 and Sb2O3, and 0 to 5% as a total quantity of F of fluoride of at least one metal element in the aforementioned metal oxides. As the case may be, the crystallized glass contains 0 to 5% of at least one coloring component selected from the group consisting of V2O5, CuO, MnO2, Cr2O3, CoO, MoO3, NiO, Fe2O3, TeO2, CeO2, Pr2O3, Nd2O3 and Er2O3. The crystallized glass further contains a main crystal of lithium disilicate, and, as the case may be, xcex1-cristobalite, xcex1-quartz, lithium monosilicate, xcex2-spodumene and the like. The size of crystal grain is 3.0 xcexcm or less.
(2) Crystallized Glass 2
The crystallized glass contains in percentages by weight 45 to 75% of SiO2, 4 to 30% of CaO, 2 to 15% of Na2O, 0 to 20% of K2O, 0 to 7% of Al2O3, 0 to 2% of MgO, 0 to 2% of ZnO, 0 to 2% of SnO2, 0 to 1% of Sb2O3, 0 to 6% of B2O3, 0 to 12% of ZrO2, 0 to 3% of Li2O, and 3 to 12% as the total quantity of F of fluoride of at least one metal element in the aforementioned metal oxides. As the case may be, the crystallized glass contains the coloring components of Cr2O3, Co3O4 and the like and the main crystal of canasite or potassium-fluoro-Richterite. The size of crystal grain is 1.0 xcexcm or less.
(3) Crystallized Glass 3
The crystallized glass contains in percentages by weight 35 to 60% of SiO2, 10 to 30% of Al2O3, 12 to 30% of MgO, 0 to 10% of ZnO, 5 to 12% of TiO2, and 0 to 8% of NiO, further contains a spinel crystal and a pyroxyene crystal as main crystals, and has a rupture modulus of 15,000 psi or more, Knoop hardness of 760 or more, and Young""s modulus of 20xc3x97106 or more.
(4) Crystallized Glass 4
The crystallized glass contains in molecular percentages 42 to 65% of SiO2, 11 to 25% of Al2O3, 15 to 33% of MgO, 5.5 to 13% of TiO2, and 0 to 4% of ZrO2, and contains a main crystal phase of xcex1-quartz solid solution and enstatite, and/or enstatite solid solution.
(5) Glass 1
The glass contains in percentages by weight 62 to 75% of SiO2, 4 to 18% of Al2O3, 0 to 15% of ZrO2, 3 to 12% of Li2O and 3 to 13% of Na2O. Alternatively, a chemical strengthening glass contains in percentages by weight 62 to 75% of SiO2, 5 to 15% of Al2O3, 4 to 10% of Li2O, 4 to 12% of Na2O and 5.5 to 15% of ZrO2, in which the weight ratio of Na2O/ZrO2 is 0.5 to 2.0 and the weight ratio of Al2O3/ZrO, is 0.4 to 2.5.
(6) Glass 2
The chemical strengthening glass contains as glass components in molecular percentages 0.1 to 30% of TiO2, 1 to 45% of CaO, 5 to 40% in total of MgO and the CaO, 3 to 30% in total of Na2O and Li2O, less than 15% of Al2O3 and 35 to 65% of SiO2.
For the purpose of enhancing shock resistance, vibration resistance and the like, a chemical strengthening process can be applied to the surface of the glass substrate by means of a low-temperature ion exchange method. The chemical strengthening method is not especially restricted as long as it is a heretofore known chemical strengthening method. For example, with respect to the glass transition point, a low-temperature chemical strengthening method or the like is preferable in which ion exchange is performed in a region which does not exceed the transition temperature. As alkali molten salt for use in the chemical strengthening, potassium nitrate, sodium nitrate or mixture nitrate of these can be used.
The substrate layer is constituted of at least one material selected from non-magnetic metals consisting of, for example, cr, Mo, Ta, Ti, W, V, B, Al and the like. When the magnetic layer is constituted mainly of Co, in order to enhance the magnetic property and the like, the substrate layer is preferably constituted of Cr simplex or Cr alloy. Also, the substrate layer is not restricted to a single layer, and can be constituted of plural identical or different layers. For example, Cr/Cr, Cr/CrMo, Cr/CrV, CrV/CrV, Al/Cr/CrMo, Al/Cr/Cr, Al/Cr/CrV, Al/CrV/CrV or another multi-layered substrate layer can be used.
As the magnetic layer, CoPt, CoCr, CoNi, CoNiCr, CoCrTa, CoPtCr, CoNiPt, or CoNiCrPt, CoNiCrTa, CoCrTaPt, CoCrPtSiO or another magnetic thin film constituted mainly of Co can be used. The magnetic layer may have a multilayered constitution (e.g., CoPtCr/CrMo/CoPtCr, CoCrTaPt/CrMo/CoCrTaPt or the like) in which the magnetic film is divided by a non-magnetic film (e.g., Cr, CrMo, CrV or the like) to reduce noise. The magnetic layer for use in a magnetic resistant head (MR head) or a great magnetic resistant head (GMR head) is constituted of the Co system alloy which includes impurity elements selected from the group consisting of Y, Si, rare earth element, Hf, Ge, Sn, Zn or oxide of these impurity elements. Alternatively, the magnetic layer has a granular structure in which Fe, Co, FeCo, CoNiPt or other magnetic particles are dispersed in a non-magnetic film constituted of ferrite system, iron-rare earth system, SiO2, BN and the like. Further, the magnetic layer may use either inner-face or vertical record format.
As the protective layer, for example, a Cr film, a Cr alloy film, a carbon film, a zirconia film, a silica film or the like can be used. The protective layer can be continuously formed together with the substrate layer, the magnetic layer and the like in an in-line sputtering device. The protective layer may be constituted of a single layer or multiple layers constituted of identical or different films. Further on the aforementioned protective layer, a protective layer other than the aforementioned protective layer can be formed. For example, on the Cr film applied is a tetra-alkoxy silane diluted with alcoholic system solvent with colloidal silica particulates dispersed therein. Further, burning is performed to form a silicon dioxide (SiO2) film.
The lubricating layer is formed by diluting a liquid lubricant of perfloropolyether (PEPE) with Freon system solvent or the like and applying the diluted PEPE onto the surface of the medium in dipping, spin coating or spraying method. If necessary, a heating process is performed.